Significant efforts have been directed to modifying the properties of known or existing materials in a manner which renders the materials suitable for use in environments which normally would adversely affect such materials. For example, one such modifying approach generally relates to coating onto a surface of a substrate material a second material having properties which differ from the underlying substrate material.
Various methods exist for coating substrate materials. A first category of coating processes is generally referred to as overlay coatings. Overlay coatings involve, typically, a physical deposition of a coating material onto a substrate. The coating material typically enhances the performance of the substrate by, for example, increasing the erosion resistance, corrosion resistance, high temperature strength, etc., of the substrate material. These overlay coatings typically extend the life of the substrate material and/or permit the use of the substrate material in a number of environments which normally might adversely affect and/or destroy the utility of the substrate material absent the placement of the overlay coating thereon.
Commonly utilized overlay coating methods include Painting, Dipping, Spraying, Spin Coating, Chemical Vapor Deposition, Hot Spraying, Physical Vapor Deposition, etc. Such methods as Painting, Dipping, Spraying and Spin Coating are readily understood to an artisan of ordinary skill in the art as widely-applicable conventional coating techniques. Chemical Vapor Deposition utilizes a chemical process which occurs between gaseous compounds when such compounds are heated. Chemical Vapor Deposition will occur so long as the chemical reaction produces a solid material which is the product of the reaction between the gaseous compounds. The Chemical Vapor Deposition process is typically carried out in a reaction chamber into which both a reactive gas and a carrier gas are introduced. A substrate material is placed into contact with the reactant and carrier gases so that reaction between the gases and deposition of the reaction solid will occur on the surface of the substrate. Chemical Vapor Deposition processes typically involve the use of corrosive gases (e.g., chlorides, fluorides, etc.) in the reaction chamber which can be quite corrosive and must be handled carefully. Accordingly, even though Chemical Vapor Deposition processes may produce desirable coatings on some materials, the equipment that is utilized typically is complicated in design and expensive to operate.
Hot Spraying techniques also exist for the placement of an overlay coating on a substrate material. The three most widely utilized Hot Spraying techniques include Flame Spraying, Plasma Spraying, and Detonation Coating.
Flame Spraying utilizes a fine powder which is contained in a gaseous stream and which is passed through a combustion flame to render the fine powder molten. The molten powder is then caused to impinge on a surface of a substrate material which is to be coated, which material is typically cold relative to the flame spray. Bonding of the coating of flame-sprayed material to the substrate is primarily of a mechanical nature. The flame-sprayed coating is usually not fully dense and, thus, is often subsequently treated by a fusing operation to densify the coating.
Plasma Spraying is somewhat similar to Flame Spraying, except that the fine powder, instead of being passed through an intense combustion flame, is passed through an electrical plasma which is produced by a low voltage, high current electrical discharge. As a result, disassociation and ionization of gases occur which results in a high temperature plasma. The high temperature plasma is directed toward a substrate material resulting in the deposition of a layer of coating material on the substrate.
Detonation Coating is a process which has some similarities to Flame Spraying, except that a desired amount of powder is directed at high velocity (e.g., about 800 meters per second) toward the surface of a substrate material which is to be coated. While the particles are being accelerated in a hot gas stream, the particles melt. Moreover, the high kinetic energy of the particles when impinging on the surface of a substrate material results in additional heat being generated, thereby assisting the coating process.
Physical Vapor Deposition coatings include, for example, Ion Sputtering, Ion Plating, and Thermal Evaporation.
In Ion Sputtering, a vacuum chamber houses a cathode electrode such that the cathode electrode emits atoms and atomic clusters toward a substrate material to result in a sputtered film or coating being deposited on the substrate.
Ion Plating of a substrate material involves the use of a heated metal source which emits metal atoms toward a substrate material which is to be coated. Specifically, an electron beam is typically utilized to excite the metal atoms from the metal source. The excited metal atoms are then directed toward the substrate material to be coated.
Thermal Evaporation also relies on the excitation of atoms from a metal source. Specifically, in a vacuum chamber, a metal source is heated so that metal atoms evaporate from the metal source and are directed toward a substrate material to be coated. The metal atoms then collect as a coating on the substrate.
A second general category of coating formation techniques is known as conversion coating techniques. In conversion coating techniques, a substrate material, typically, is involved in a chemical reaction which modifies the composition and/or microstructure of the surface of the substrate. These conversion coating techniques also can result in desirable surface morphology modification of substrate materials. Typical examples of conversion coating techniques include Pack Cementation and Slurry Cementation. Exemplary of specific conversion coating compositions which may be applied to substrates are conversion coating techniques referred to as chromating and aluminizing, whereby a coating composition comprising such materials as chromium or aluminum is applied to a surface of a substrate and reacted with the substrate upon, for example, heating, etc.
Pack Cementation and Slurry Cementation utilize diffusion of one or more materials to form a surface coating. Specifically, in each of these processes, a substrate material is contacted with a metal source material such that a metal from the metal source material may diffuse into the substrate material and/or a component of the substrate material may diffuse toward the metal source material. Specifically, for example, in Pack Cementation, a substrate material is buried within a powder mixture which comprises, typically, both a metal which is to react with the substrate material and an inert material. A carrier gas is then induced to flow into the powder mixture so that the carrier gas can carry metal atoms from the metal powder to the surface of the substrate and deposit the metal atoms thereon. In Slurry Cementation, a composition typically is coated onto a surface of a substrate material prior to conducting the diffusion process. Both Pack Cementation and Slurry Cementation typically occur in a retort or vacuum furnace at elevated temperatures, and the carrier gas is free to transport metal atoms from the metal powder to the surface of the substrate material. Typical carrier gases include the halogen gases. Many different approaches to Pack Cementation have been made; however, most of these approaches utilize the above-discussed steps.
Conversion coatings techniques have also been carried out utilizing starting materials other than the materials discussed above with respect to Pack Cementation and Slurry Cementation. Materials such as organic resins and polymers have also been demonstrated to provide effective coatings against, for example, oxidation and corrosion under specified environmental conditions.
Protective ceramic coatings on, for example, carbon/carbon composites, graphite, carbon fibers and other oxidizable materials, formed from preceramic polymers which can be converted to ceramic upon heating have been described. U.S. Pat. No. 5,198,488 (Patent '488), in the name of Leonard M. Niebylski, is directed to the preparation of preceramic compositions which may be used to provide, among other applications, oxidation-resistant coatings on carbon/carbon composites, graphite, carbon fibers and other normally oxidizable materials by dispersing about 0.1-4 parts by weight of silicon boride in one part by weight of a polysilazane in solution in an organic solvent. The preceramic compositions are coated onto the oxidizable materials and heated to temperatures of about 675.degree.-900.degree. C. to pyrolyze the preceramic compositions to ceramic coatings. Patent '488 also teaches that for high temperature (i.e., higher than 800.degree. C.) oxidation protection, the pyrolysis step is followed by thermal treatment of the coated substrate at about 1075.degree.-1250.degree. C. in an atmosphere containing not more than a minor amount of oxygen.
U.S. Pat. No. 5,196,059 (Patent '059), also in the name of Niebylski, is directed to preceramic compositions utilized, among other applications, to provide heat, abrasion and oxidation resistant ceramic coatings, prepared by dispersing about 0-3 parts by weight of aluminum-silicon eutectic, about 0-4 parts by weight of silicon carbide, about 1.5-5 parts by weight of silicon boride, and about 0.4-5 parts by weight of silicon metal in a solution of one part by weight of a polysilazane in an organic solvent. The formation of ceramic coatings in Patent '059 is substantially as set forth above with respect to Patent '488.
U.S. Pat. No. 5,194,338 (Patent '338), also in the name of Niebylski, is directed to preceramic compositions utilized, among other applications, to provide protective ceramic coatings on normally oxidizable materials prepared by dispersing about 0.4-3.0 parts by weight of a ceramic powder selected from silicon carbide, silicon nitride, and mixtures thereof, with one another and/or with up to about 90 percent by weight of (1) a metal boride or (2) a mixture of a metal boride and zirconium metal in one part by weight of a polysilazane. The formation of ceramic coatings in Patent '338 is substantially as set forth above with respect to Patent '488.
U.S. Pat. No. 5,258,224, in the names of Conrad J. Langlois, Jr., et al., is directed to preceramic compositions which are useful to provide protective ceramic coatings on normally oxidizable materials and which coating compositions are obtained by dispersing solid particles comprising aluminum nitride particles in an organic solvent solution of a polysilazane. Ceramic coatings derived from such dispersions may serve as intermediate strata in multilayer ceramic coatings over substrates, such as carbon/carbon substrates, to further protect such substrates, even when exposed to humidity.
The above-discussed coating compositions and techniques have been briefly addressed herein to give the reader a general understanding of the art. However, it should be understood that specific variations to the above-discussed compositions and techniques exist. Specifically, each of the coating compositions and/or processes discussed above are discussed in detail in readily available sources, including textbooks, conference proceedings, and patents. For further information relating to the detail of these processes, the reader is encouraged to consult the literature referred to above. However, even from the brief discussions above, it should be clear that each of the techniques suffers from various limitations. For example, in the overlay coating techniques, the physical deposition of a coating onto a substrate material does not insure an acceptable interface between the substrate and the coating. Specifically, because most of the overlay coating techniques simply rely on the use of a physical bonding between the coating and the substrate, the coating may not adhere adequately to the substrate. Accordingly, the purpose of the coating may be compromised completely. Additionally, the overlay coating processes typically depend on the use of somewhat complex deposition equipment. For example, Chemical Vapor Deposition requires the use of relatively complicated control means for controlling the rate of flow of reactive and carrier gases in a reaction chamber, the ability to handle corrosive gases (e.g., fluorides, chlorides), etc.
Moreover, with regard to the so-called conversion coating techniques which are formed by, for example, Pack Cementation and Slurry Cementation techniques, the coatings achieved on substrate materials may not be uniform due to the inclusion of solid materials or porosity which result from exposure of the substrate to either of or both of the powder metal source and/or inert materials utilized in the Pack Cementation or Slurry Cementation processes. Still further, many of the Pack Cementation and Slurry Cementation techniques may require the use of somewhat complex equipment.
Further, with regard to the techniques discussed in connection with the preceramic compositions of Niebylski, it is noted that these techniques include the use of specific controlled atmospheres to obtain desirable oxidation resistance at high temperatures, thus requiring the use of somewhat complex equipment. Moreover, Langlois, Jr., et al. teaches the use of specified intermediate strata compositions in multilayer ceramic coatings to enhance protection of substrates. The use of such multicompositional layers within coatings increases not only the cost of making such coatings, but also introduces the potential for incompatibility of coating layers due to, for example, coefficient of thermal expansion mismatch between layers, etc.
Accordingly, a long-felt need has existed for compositions which may be used to provide enhanced protection against, for example, oxidation, hydrolysis, etc., when applied to oxidizable substrates, without the requirement for expensive and/or complex processes.